Epileptic seizures are the outward manifestation of excessive and/or hypersynchronous abnormal activity of neurons in the cerebral cortex. Many types of seizures occur. The behavioral features of a seizure reflect function of the portion of the cortex where the hyper activity is occurring. Seizures can be generalized and appearing to involve the entire brain simultaneously. Generalized seizures can result in the loss of conscious awareness only and are then called absence seizures (previously referred to as “petit mal”). Alternatively, the generalized seizure may result in a convulsion with tonic-clonic contractions of the muscles (“grand mal” seizure). Some types of seizures, partial seizures, begin in one part of the brain and remain local. The person may remain conscious throughout the seizure. If the person loses awareness, the seizure is referred to as a complex partial seizure.
A number of techniques are known to treat seizures including, for example, drug therapy, drug infusion into the brain, electrical stimulation of the brain, electrical stimulation of the nervous system, and even lesioning of the brain. U.S. Pat. No. 5,713,923 entitled Techniques of Treating Epilepsy by Brain Stimulation and Drug Infusion” generally discloses such techniques in the background section and specifically discloses techniques for drug infusion and/or electrical stimulation to treat epilepsy. This patent is incorporated herein by reference in its entirety.
U.S. Pat. No. 5,025,807 entitled “Neurocybemetic Prosthesis” and its parentage (U.S. Pat. Nos. 4,867,164 and 4,702,254) (all three patents are collectively referred to herein as the “Zabara patents”) disclose techniques for electrical stimulation of the vagus nerve. These Zabara patents generally disclose a circuit-based device that is implanted near the axilla of a patient. Electrode leads are passed from the circuit device toward the neck and terminate in an electrode cuff or patch on the vagus nerve.
The neuro-cyb emetic prosthesis (NCP) is the primary vagus nerve stimulation (VNS) system that is presently available. This presently available VNS treatment technique for the treatment of epilepsy, however, has limited therapeutic efficacy and exerts clear but variable chronotropic effects on the human heart. See Handforth et. al., “Vagus Nerve Stimulation Therapy for Partial Onset Seizures: A Randomized Active Control Trial,” J. Neurology, Vol. 51, pp. 48-55 (1998); Han et al, “Probable Mechanisms of Action of Vagus Nerve Stimulation in Humans with Epilepsy: Is the Heart the Window into the Brain?” AES Proceedings, p. 83 (1997); Frei et al., “Effects of Vagal Stimulation on Human EEG,” AES Poceedings, p. 200 (1998) (each of these references are incorporated herein by reference in their entireties). With regard to the heart, vagus nerve stimulation has the side-effect of altering the heart rate. See Frei et al. “Effects of Vagal Stimulation on Human ECG,” Abstract from the Annual Meeting of the American Epilepsy Society, Vol. 39, Supp. 6 (1998), which is incorporated herein by reference in its entirety. Typically, activation of the device and stimulation of the vagus nerve causes the heart to experience a significant drop in heart rate. For example, FIG. 1A is graph illustrating the effects of vagus nerve stimulation on the heart rate for a patient. In this Figure, the horizontal axis represents time and the vertical axis represents the normalized heart rate. A value of 1 in this graph indicates that the instantaneous heart rate (IHR) at that point in time is equal to the median IHR for the current vagus nerve stimulator (VNS) device cycle (i.e., for the current 5½ minute window). The graph shows that during vagus nerve stimulation from time 0 to 50, the heart rate drops to as low as 0.8 of its background rate. Similarly, FIG. 1B is a graph of the instantaneous heart rates (defined herein) of a patient as a function of time over an 8 hour period. The sharp drops that occur periodically along the bottom of the graphed line correspond to times when the vagus nerve stimulation device is reset or turned “on”. These sharp drops illustrate the effect that vagus nerve stimulation has on the heart. Notably, the Zabara patents recognize that the heart rate slows as a result of the stimulation. This effect that vagus nerve stimulation has on the heart is undesirable due to negative short- or long-term effects on the patient. For example, the heart may become less adaptable to stresses due to the vagus nerve stimulation, which may lead to arrhythmia, asystole (heart stoppage), and possibly even to sudden death. See Asconape et al, “Early Experience with Vagus Nerve Stimulation for the Treatment of Epilepsy; Cardiac Complications, AES Proceedings, p. 193 (1998) (incorporated herein by reference in its entirety).
The relative lack of efficacy and the adverse effects of the VNS are attributable in part to inadequate stimulation. Specifically, the NCP does not change the electro-encephalogram (EEG) reading. See Salinsky et al. “Vagus Nerve Stimulation Has No Effect on Awage EEG Rythms in Humans,” J. Epilespia, Vol. 34 (2), p.299-304 (1993). Adequate stimulation of the vagus nerve induces either synchronization or desynchronization of brain rhythms depending on the stimulation parameters used. See Michael H. Chase et al., “Afferent Vagal Stimulation: Neurographic Correlates of Induced EEG Synchronization and Desynchronization,” Brain Research pp. 236-249 (1967); Chase et al, “Cotical and Subcortical Patterns of Response to Afferent Vagul Stimulation,” Experimental Neurology, Vol. 16, pp. 36-49 (1966). EEG desynchronization requires selective activation of slow conducting nerve fibers. This state of desynchronization does not favor the occurrence of seizures and is therefore preferred for this specific therapeutic purpose. The absence of EEG changes in humans during VNS suggests stimulation is inadequate and this in turn may explain its relatively low therapeutic value. See Handforth et. al., “Vagus Nerve Stimulation Therapy for Partial Onset Seizures: A Randomized Active Control Trial,” J. Neurology, Vol. 51, pp. 48-55 (1998).
In addition, VNS provides non-selective bi-directional nerve fiber activation. In general, the VNS stimulation affects the brain (a desirable target) and also the viscera, including the heart (undesirable targets). Accordingly, VNS causes alterations in the heart electrocardiogram (EKG) reading. Given the shape of the pulse, its biphasic nature and the intensity settings available in the NCP, selective stimulation of slow conducting nerve fibers (a necessary condition for EEG desynchronization) is highly unlikely with this device.
Further, the NCP provides indiscriminate timing for stimulation of the heart. Cardiac arrest can result from stimulation of the heart during vulnerable phases of its cycle. See Jalife J, Anzelevitch C., “Phase resetting and annihilation of pacemaker activity in cardiac tissue,” Science 206: 695-697 (1979); Jalife J, Anzelevitch C., “Pacemaker annihilation: diagnostic and therapeutic implications,” Am. Heart J. 100: 128-130 (1980); and Winfree AT., “Sudden Cardiac Death: A Problem in Topology,” Sci Am 248: 144-161 (1983). VNS can cause cardiac arrest because the timing of stimulation does not take into account the phase or state of the cardiac cycle.
Accordingly, it is an object of the invention to provide a technique for controlling or preventing epilepsy via stimulation of the vagus nerve with minimized effect on the heart rate. It is another object of the invention to provide a technique for adjusting the vagus nerve stimulation to minimize its affect on the heart rate. It is another object of the invention to provide stimulation of the vagus nerve while maintaining the heart rate at a preset rate. It is a further object to minimize the risk of cardiac arrest in patients receiving VNS by delivering stimuli at times in the heart cycle which cause no or minimal adverse effects on rhythms generation or propagation. Other objects of the present invention will become apparent from the following disclosure.